Such apparatuses are to be found, for example, in measuring devices of process measurements technology. These measuring devices are frequently employed in automation and process control technology for measuring a process variable, such as e.g. limit level, fill level, dielectric constant or some other physical and/or chemical process variable during the course of a process. Produced and sold by the assignee, for example, are measuring devices under the marks Levelflex and Multicap, which are primarily designed for determining and/or monitoring fill level of a medium in a container. In one of many travel-time measuring methods, for example, according to the method of guided microwaves, time-domain reflectometry, also called the TDR measuring method, a high frequency pulse is transmitted along a Sommerfeld or Goubau waveguide, or coaxial waveguide, to then be partially reflected back at a discontinuity of the DK (dielectric constant) value of the medium surrounding the waveguide. From the time difference between transmitting of the high frequency pulse and the reception of the reflected, echo signal of the medium, fill level can be ascertained. The FMCW-method (Frequency Modulated Continuous Wave), in the case of which the frequency of a continuous measurement signal changes and the distance is measured by the frequency difference of the transmitted measurement signal compared to the reflected measurement signal, is likewise usable in connection with the above principle of measurement.
Another measuring principle used in a large number of measuring methods for ascertaining fill level in a container concerns measuring change in capacitance of a capacitive measurement assembly involving a measuring probe contacting the medium and a wall of the container or a reference electrode, when the degree of immersion of the probe in the medium, respectively fill level of the medium in the container, changes.
In both methods described above, a medium-contact fill-level measuring method is involved, wherein a measuring probe comes into direct contact with the medium to be measured. Such measuring probe is usually secured in a container at a process connection, opening or nozzle, so that the measuring electronics remains outside of the process, i.e. not in contact with the medium, and the measuring probe is integrated in the process. In the following references, construction of such measuring probes, respectively waveguides, and the coupling of the measurement signals into these measuring probes are discussed.
In EP 1 069 649 A1, the waveguide for a fill level measuring device is shown having a simple construction, which combines the advantages of single wire and known multi-wire waveguides, in that it shows no interaction with installed objects in the container and can be cleaned of accretions and deposits in simple manner. This is achieved by surrounding the multi-wire waveguide in the process at least partially with a dielectric medium, so that no accretion can form between the individual waveguides.
In DE 100 27 228 A1, a form of embodiment of the coupling of high frequency signals into a surface waveguide is shown. In this document, coupling is embodied in such a manner that disturbance voltages are effectively bled off, since the surface waveguide is connected via a coupling to ground potential.
In DE 100 19 129 A1, several types of coupling units are disclosed, which are so lengthened that the structural parts lie outside of the area in which electromagnetic energy is radiated. In this way, the influence, which a structural part or accretion formation on the sensor has on accuracy and sensitivity of measurement, can be largely eliminated.